in the long term, yes, as you can't do the higher paper of a subject for your leaving cert if you didn't do higher for junior cert, so you can earn less points, because for example 90 to 100 percent in ordinary level leaving cert is 56 points per subject, while in a higher subject it is a full 100 points. Basically its better to get a C in higher than an A in ordinary, if you're capable of course. If you're not, it's not worth it.

OH, apologies, i only read the title of your question, sorry, i thought you may have been asking the pont of doing higher level in the first place. No, if you're doing ordinary level already do not study anything higher for that subject, unless some topics intertwine i guess.